Many composite materials are made by combining reinforcing fibers with resinous or plastic matrix material. A number of different manufacturing techniques are used to produce the composite parts. One of the least expensive techniques is the pultrusion process. As the name would suggest, pultrusion is a combination of a pulling and an extrusion process. The reinforcement structure, which may be fibers, cloth, or other forms, is continuously pulled from spools and impregnated with the matrix material. The combined matrix impregnated reinforcement fiber is formed into its final shape by pulling it through a heated die. The final curing of the composite also occurs in the downstream portion of the die.
A critical stage of the pultrusion process is the application of the matrix material to the reinforcement structure. The object is to impregnate the reinforcement structure, both around the various discrete elements, and throughout each discrete element with the matrix material as fully as possible. Voids and air pockets are undesirable. Increased numbers and size of voids generally results in less desirable structural properties of the composite. High void content reduces various properties, including strength and resilience and makes it more difficult to machine the composite.
It is known to apply the matrix material to the reinforcement material using a process known as a wet bath. A typical wet bath system is shown schematically in FIG. 1. Reinforcing material of fiber 2 is maintained on racks 4, or spindles or other suitable support. Additional reinforcing material of cloth 6 is maintained on racks 8 and spindles 10. The fiber strands 2 pass through preforming guides 12 to associate the fibers in a preliminary grouping. The fiber strands 2 and the cloth bolts 6 pass together through first material guide 14 and come together in closer association. A tank 16 holds a volume of liquid matrix material, such as resin. The associated reinforcement tow of fibers 2 and cloth 6 is drawn through the resin bath and resin soaks into the reinforcement material, just as water soaks into a towel dropped into a bath tub. The wetted tow 18 is drawn through rollers 20 and a second material guide 22, which further shapes the composite. The composite tow is shaped within pultrusion die 24, described in more detail below, where it is also cured by action of heaters 26.
Various arrangements for advancing the tow are known. FIG. 1 shows a basic configuration. A pair of hydraulic rams 28, 30 are each attached to a clamping pulling mechanism 32, 34 respectively. The pulling mechanisms act in alternation, first one clamping down on the tow with its movable element 36, and then activating its hydraulic ram to pull the tow away from the beginning of the line. Simultaneous with this action, the other mechanism 34 has disengaged its movable element 38 and reversed its hydraulic ram 30 to bring the pulling mechanism 34 back to the beginning position. Thus pulling mechanism 34 will be ready to engage the tow when pulling mechanism 32 has reached the end of its travel. Downstream, cutoff saw 40 cuts of the cured composite material in whatever lengths are desired. As an alternative, a system of roller pullers can be used to move material through the system.
The wet bath process has a number of drawbacks. Only a limited number of matrix materials can sit in the bath for the long periods of time needed to be soaked up without spontaneously curing or hardening. Further, due to the relatively long period of time it takes for the resin to soak into the reinforcement material, the line can not exceed a certain speed. The line is a series of continuous operations and the speed of the slowest operation controls the pace of the entire system. When the resin sits in the open tank for a period of time, it is subject to contamination. Finally, the large, substantially open resin tank releases potentially dangerous vapors into the factory atmosphere.
Another known technique, called the direct resin injection technique, avoids some of the problems of the wet bath method. The direct resin injection technique is shown schematically in FIG. 2. Most of the elements are the same as in the wet bath technique, and are referred to by like reference numerals. In the place of the wet bath, the direct injection technique may optionally use a preheater 17. The preheater 17 heats the reinforcement material to facilitate impregnation with matrix material. The reinforcement material is drawn through pultrusion die 24 (discussed in more detail below). Within the die 24, resin is introduced through port 42, from pressurized tank 44 and line 46 or other resin pumping mechanisms.
FIG. 3 shows schematically resin injection pultrusion die 24 of the prior art. Prior art dies for wet bath pultrusion do not contain an injection region. The associated reinforcement material of fibers 2 and cloth 6 enter the die at entrance port 50. The initial portion of the die immediately in the vicinity compresses the bulk of the associated reinforcement material. Downstream from the initial, substantially highly compressed fiber region is a cavity 52 of a larger volume. Injection ports 42a and 42b provide matrix material resin or plastic at elevated pressure, and possibly elevated temperature, into the cavity 52. The resin is delivered from line 46. A valve 54 may be provided to adjust pressure or to facilitate cleaning. The cavity 52 is of a generally teardrop shape, with the portion of larger cross section in the upstream direction and the portion of smaller cross section in the downstream direction. As the resin is pumped into the cavity under pressure, it impregnates the bundle of compressed reinforcement material. It occupies the spaces between various fibers and elements of the reinforcement material, and also the spaces within the fibers themselves. The pulling action of the impregnated composite, toward the narrow end of the cavity 52, and the taper itself, combine to create a hydraulic pressure, which is typically higher than the matrix material injection pressure. The majority of trapped air in the composite is forced upstream due to the hydraulic pressure, and eventually exits the die through entrance port 50. Heaters 26 keep the die at an elevated temperature so that the resin cures at an appropriate rate. The resin injection pressure and the degree of compression of the reinforcement material determine the speed of the resin flow and hence the pulling speed of the product.
The direct resin injection method overcomes many of the drawbacks of the wet bath method. It provides a generally cleaner work environment and increases the number of matrix materials that can be used. However, the void content of direct injected composites is unsatisfactorily high, often as much as 20% by volume.
Voids are created by trapped air bubbles. This is especially true when "straight rovings" or mats with fiber orientations at 90.degree. to the pulling direction are used. Air is also usually present in the resin due to the normal methods of mixing in the resin catalysts or other additives. It is not normal procedure to degas the resin prior to processing because the time required to do this is usually longer than the "pot life" of the catalyzed resin. "Pot life" refers to the length of time the resin can sit in liquid form at the applied temperature and remain liquid and uncured. Continuous wetting of the reinforcement fibers depends on the ability to initially penetrate transversely through the fibers and completely wet out the tows. Then the resin can propagate longitudinally upstream toward the entrance the die as the fiber tow passes through the cavity. Even in the compressed state, fluffy type roving (such as Nordic 4000 Spunnrovings or FGI Texstrand) still has a high degree of resin permeability making these products highly conducive to this type of processing. It is believed that this is due to the criss-crossing of neighboring filaments within a strand. Straight rovings (sometimes called gun rovings) have highly parallel filaments within the strand and pack very closely even if relatively little external pressure is applied (low compressibility but high density). Thus, they are less conducive to this type of processing.
Applying increased resin pressure is not always sufficient to cause the resin to penetrate the strand. The viscosity of the resin is often so high that the resin coats the outside of the strand and creates a viscous shell. The external resin pressure exaggerates the problem by compacting the strand.
To date, the processing of straight rovings and reinforcement materials (mats) made with straight rovings has proven too difficult for processes using the injection pultrusion method.
The main requirements for the design of an injection chamber are to achieve transverse penetration and removal of as much of the voids as possible. The chamber is generally made to be about twice the thickness of the part and have the symmetric teardrop shape as shown in FIG. 3. It is generally about 4 inches long and has a 2 inch long primary compression region preceding the chamber. The resin flows into the chamber through holes placed in the widest part of the chamber on both sides. The number of holes is determined by the designers' judgement from experience and on the complexity of the part. Generally, greater numbers of holes improves processing.
For the reasons discussed above, it is desirable to further reduce the void content of composites below that produced by the direct injection method.